Are there any instances that disable downvoting?
Yes, I think Beehaw for example has no downvotes (or doesn't show them).
Beehaw and my instance are the only ones that I know of
https://lemmy.one/
This page, "Awesome Lemmy instances", has a big list of instances with some info, including whether they allow downvotes. I'm not sure if it's up to date though.